Input devices for modern graphics display systems permit the user to alter a displayed image. One such input device is known as a trackball. A conventional trackball includes a rigid ball that is rotatably mounted within a housing. A portion of the ball extends through an opening in the housing and is exposed to the user.
Motion sensors are mounted within the housing to detect movement of the ball. Whenever a user rotates the ball by manipulating its exposed portion, the motion sensors generate output signals that are representative of the direction and magnitude of the ball rotation. The output signals are supplied to the display controller and the displayed image is changed (for example, rotated) in response to those signals.
The housings of prior trackballs are configured so that more than half of the ball is enclosed therein. Further, the opening through which the exposed portion of the ball extends has a diameter that is smaller than that of the ball. The ball remains secured to the housing because it cannot pass through the relatively smaller diameter opening.
The just-described means for securing the ball to the trackball housing, although effective, necessarily limits the amount of surface area that can be exposed for manipulation by the user. Oftentimes, less than one-third of the ball surface area is exposed. This limited exposed surface area of the ball makes the ball difficult to grasp. Accordingly, the user finds it difficult to control fine rotational movement of the ball to produce correspondingly fine changes in the displayed image.